Successful electroplating of some metals and alloys depends critically on the control of species other than the metal cations in the plating solution Among these species are the "additives" or "addition agents" that must be included to give the necessary properties of the deposited material. The addition agents are notoriously difficult to control. A primary difficulty is that they are often purchased from outside vendors, and therefore the identities of the addition agents are often not known. Even for known compounds, good techniques may not exist for analysis in the matrix of the plating solution. Additionally, the additives are usually mixtures of different kinds of chemicals and may be present at very low concentrations.
The copper system demonstrates the current state of the art of additive analysis. Copper deposition from acid CuSO.sub.4 is a relatively old technology. There have been years of work on the analysis of the additives in the Cu plating solution. The monitoring methods are semi-quantitative, at best. The amount of Cu plated in cyclic voltammetry, taken to be measurable from the stripping charge, was shown to give some measure of the additive levels in the solution. See, for example, R. Haak, C. Ogden, and D. Tench, Plating, 64(4), 52 (Apr 1981); R. Haak, C. Ogden, and D. Tench, Plating, 65(3), 62 (Mar. 1982). This CV method is not an analytical procedure as the term is generally understood: it is not specific for a given chemical compound, and the relationship between measured charge and solution concentration is not direct. In addition, the CV measures the aggregate effects of all of the additive components. Some effort has been made to use the technique to determine the individual components of a multi-component additive (see W. O. Freitag, C. Ogden, D. Tench, and J. White, Plating, 70(10), 55 (Oct. 1983)), but it is questionable whether such a procedure can be the basis of plating solution control.
A quantitative analytical technique, liquid chromatography, is available for some of the components of some electroplating solutions (see S. S. Heberling, D. Campbell, and S. Carson, PC Fab, Aug. 1989, p. 72). One of the more useful examples is the HPLC analysis of the MD (or Carrier) component of the SelRex Cubath M-Hy acid copper plating solution. Even in this particular plating solution, however, some of the additive components cannot be separated or detected with HPLC. The chromatographic analysis thus allows only partial control of the plating process.
The present inventors have developed a method, disclosed in U.S. patent application Ser. No. 07/701,278, filed May 16, 1991, the entire disclosure of which is incorporated herein by reference thereto, that allows the separate determinations of the MD (Carrier) and M-Lo (Leveller/Ductilizer) components of SelRex Cubath M-Hy. An improved cyclic voltammetric method can be used to determine M-Lo only. The combination of HPLC and CV can thus be used to monitor and control both of the components of the acid-copper additives. The common CV technique measures the amount of Cu deposited as a function of the addition of known volumes of an unknown solution to an additive-free "basis solution." A calibration with a solution of known concentration allows the calculation of the additive concentration in the unknown. The modification that allows CV to be sensitive only to one component of the additive (in this case, M-Lo) is the incorporation in the basis solution of all of the other solution components, including the other additive components (in this case, MD).
The example of Cu shows the formidable problems presented in monitoring and control of plating solutions. The Cu system is, however, much better understood than some other systems of practical interest. The present invention addresses the issue of control of PbSn plating systems. These alloys also require the use of appropriate additives in order to obtain the necessary deposit properties.
The technology of PbSn plating is presently evolving to a system of novel chemistry--solutions based on methane sulfonic acid. PbSn electroplating techniques generally are described in E. K. Yung, and I. Turlik, "Electroplated Solder Joints for Flip-Chip Applications," IEEE Trans. on Components, Hybrids, and Manufacturing Technology, Vol. 14, No. 3, Sept. 1991, the disclosure of which is incorporated herein by reference. The MSA-based PbSn plating solutions give good alloy deposits and eliminate some of the environmental and health hazards associated with the commonly used fluoroborate-based solutions. Since they have been only recently introduced, however, they have not been as extensively studied. Monitoring techniques have not been extensively developed. The vendors supply specifications for analytic procedures for all of the solution components. The present inventors have found these procedures to be unacceptable for the monitoring of the additive system in the solution being used, namely, the LeaRonal Solderon SC solution. The present invention describes a new, better technique for determining the SC levels in PbSn plating solutions.
The present invention overcomes the deficiencies and problems associated with the conventional monitoring technology.